U.S. patent application number 11/739396 was filed with the patent office on 2007-10-25 for electronic device.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. Invention is credited to Shingo MASUKO, Ryoichi OHARA, Kenya SANO, Naoko YANASE, Takaaki YASUMOTO.
Application Number | 20070247260 11/739396 |
Document ID | / |
Family ID | 38618963 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070247260 |
Kind Code |
A1 |
YANASE; Naoko ; et
al. |
October 25, 2007 |
ELECTRONIC DEVICE
Abstract
An electronic device includes: a substrate; a first film
provided on a major surface of the substrate; and a crystalline
second film covering at least a part of the end surface and
provided on the first film and the substrate. The end surface has
an inclined surface which is inclined to the major surface of the
substrate. The inclined surface has a curved surface whose slope
becomes gentle with getting closer to the substrate.
Inventors: |
YANASE; Naoko;
(Kanagawa-ken, JP) ; YASUMOTO; Takaaki;
(Kanagawa-ken, JP) ; OHARA; Ryoichi;
(Kanagawa-ken, JP) ; SANO; Kenya; (Kanagawa-ken,
JP) ; MASUKO; Shingo; (Kanagawa-ken, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Tokyo
JP
|
Family ID: |
38618963 |
Appl. No.: |
11/739396 |
Filed: |
April 24, 2007 |
Current U.S.
Class: |
333/187 ;
333/189 |
Current CPC
Class: |
H03H 3/02 20130101; H03H
9/02149 20130101; H03H 9/174 20130101; H03H 9/02133 20130101 |
Class at
Publication: |
333/187 ;
333/189 |
International
Class: |
H03H 9/54 20060101
H03H009/54; H03H 9/58 20060101 H03H009/58 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2006 |
JP |
2006-120881 |
Claims
1. An electronic device comprising: a substrate; a first film
provided on a major surface of the substrate and having at least
one end surface; and a crystalline second film covering at least a
part of the end surface and provided on the first film and the
substrate, the end surface having an inclined surface which is
inclined to the major surface of the substrate, and the inclined
surface having a curved surface whose slope becomes gentle with
getting closer to the substrate.
2. The electronic device according to claim 1, wherein the curved
surface is a concave surface.
3. The electronic device according to claim 1, wherein the inclined
surface has a convex portion protruding toward the second film.
4. The electronic device according to claim 1, wherein the second
film is a polycrystal which is oriented in a thickness
direction.
5. The electronic device according to claim 1, further comprising
an upper electrode provided on the second film, wherein the
substrate has a cavity, the first film forms a lower electrode, and
the second film is made of one selected from the group consisting
of aluminum nitride (AlN), zinc oxide (ZnO) and lead zirconate
titanate (PZT).
6. The electronic device according to claim 1, wherein a curvature
radius of the inclined surface is larger than a thickness of the
second film.
7. The electronic device according to claim 1, wherein an angle at
an upper end of the inclined surface is substantially larger than
135.degree..
8. An electronic device comprising: a substrate; a first film
provided on a major surface of the substrate and having at least
one end surface; a crystalline second film covering at least a part
of the end surface and provided on the first film and the
substrate; and an upper electrode provided on the second film, the
end surface having an inclined surface which is inclined to the
major surface of the substrate, and the inclined surface having a
curved part at its upper end, a slope of the curved part being
gentle with getting closer to the upper electrode.
9. The electronic device according to claim 8, wherein the inclined
surface has a concave surface which is provided under the upper
end.
10. The electronic device according to claim 8, wherein the second
film is a polycrystal which is oriented in a thickness
direction.
11. The electronic device according to claim 8, wherein the
substrate has a cavity, the first film forms a lower electrode, and
the second film is made of one selected from the group consisting
of aluminum nitride (AlN), zinc oxide (ZnO) and lead zirconate
titanate (PZT).
12. The electronic device according to claim 8, wherein a curvature
radius of the inclined surface is larger than a thickness of the
second film.
13. The electronic device according to claim 8, wherein the
inclined surface having a curved surface whose slope becomes gentle
with getting closer to the substrate.
14. An electronic device comprising: a substrate; a first film
provided on a major surface of the substrate and having at least
one end surface; and a crystalline second film covering at least a
part of the end surface and provided on the first film and the
substrate, the end surface having an inclined surface which is
inclined to the major surface of the substrate, and the inclined
surface having a convex portion protruding toward the second film,
the convex portion being provided between an upper end and lower
end of the inclined surface.
15. The electronic device according to claim 14, wherein an angle
of a slope of the inclined surface has a relative maximum.
16. The electronic device according to claim 14, wherein a concave
surface is provided under the convex portion on the inclined
surface.
17. The electronic device according to claim 14, wherein the second
film is a polycrystal which is oriented in a thickness
direction.
18. The electronic device according to claim 14, wherein an angle
at an upper end of the inclined surface is substantially larger
than 135.degree..
19. The electronic device according to claim 14, wherein a
curvature radius of the inclined surface is larger than a thickness
of the second film.
20. The electronic device according to claim 14, wherein the
substrate has a cavity, the first film forms a lower electrode, and
the second film is made of one selected from the group consisting
of aluminum nitride (AlN), zinc oxide (ZnO) and lead zirconate
titanate (PZT).
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.2006-120881,
filed on Apr. 25, 2006; the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to an electronic device, and more
particularly to electronic devices such as a thin film bulk
acoustic resonator and a MEMS.
[0004] 2. Background Art
[0005] In recent years, electronic devices, such as a MEMS (Micro
Electro Mechanical System) device and a thin film bulk acoustic
resonator (FBAR) which integrate an acceleration sensor or a
pressure sensor on a silicon substrate, are developed and their
practical applications are expected.
[0006] In these electronic devices, where a first film is provided
on a major surface of a supporting substrate, furthermore a second
film is provided so as to cover the supporting substrate and an end
portion of the first film, the end portion of the first film being
substantially perpendicular to the major surface of the supporting
substrate lowers "step coverage". Then, problems of cracks and
subsidiary fractures in the second film are caused.
[0007] For example, for FBAR, a lower electrode is provided on the
supporting substrate having a cavity and a piezoelectric film is
provided on the electrode. However, cracks or subsidiary fractures
at a step portion formed at the end of the lower electrode
deteriorate a piezoelectric characteristic.
[0008] Contrary, it is disclosed that the end portion of the lower
electrode is tapered and an angle between the tapered plane and the
major surface of the supporting substrate is
5.degree..about.30.degree. (U.S. patent application Publication No.
2004/0263287A1).
[0009] However, after studies by present inventors, it is revealed
that cracks or fractures have a propensity to be caused due to
produced stresses in the second film formed over the lower end of
the tapered plane where the tapered plane has a flat shape.
SUMMARY OF THE INVENTION
[0010] According to an aspect of the invention, there is provided
an electronic device including: a substrate; a first film provided
on a major surface of the substrate and having at least one end
surface; and a crystalline second film covering at least a part of
the end surface and provided on the first film and the substrate,
the end surface having an inclined surface which is inclined to the
major surface of the substrate, and the inclined surface having a
curved surface whose slope becomes gentle with getting closer to
the substrate.
[0011] According to an aspect of the invention, there is provided
an electronic device including: a substrate; a first film provided
on a major surface of the substrate and having at least one end
surface; a crystalline second film covering at least a part of the
end surface and provided on the first film and the substrate; and
an upper electrode provided on the second film, the end surface
having an inclined surface which is inclined to the major surface
of the substrate, and the inclined surface having a curved part at
its upper end, a slope of the curved part being gentle with getting
closer to the upper electrode.
[0012] According to an aspect of the invention, there is provided
an electronic device including: a substrate; a first film provided
on a major surface of the substrate and having at least one end
surface; and a crystalline second film covering at least a part of
the end surface and provided on the first film and the substrate,
the end surface having an inclined surface which is inclined to the
major surface of the substrate, and the inclined surface having a
convex portion protruding toward the second film, the convex
portion being provided between an upper end and lower end of the
inclined surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic cross section showing an electronic
device according to a first embodiment of the invention.
[0014] FIG. 2A is a top view of the electronic device of the
embodiment, and FIG. 2B is a bottom view thereof.
[0015] FIG. 3 is a schematic cross section showing a first specific
example of the tapered portion in FIG. 1.
[0016] FIG. 4 is a schematic cross section showing a first
comparative example of the tapered portion.
[0017] FIG. 5 is a schematic cross section showing a second
specific example of the tapered portion.
[0018] FIG. 6 is a schematic cross section showing a third specific
example of the tapered portion.
[0019] FIG. 7 is a schematic cross section showing a fourth
specific example of the tapered portion.
[0020] FIG. 8 is a schematic cross section showing a second
comparative example of the tapered portion.
[0021] FIG. 9 is a schematic diagram showing a process of
manufacturing FBAR of the first embodiment.
[0022] FIG. 10 is a schematic diagram showing a process of
manufacturing FBAR of the first embodiment.
[0023] FIG. 11 is a schematic cross section showing the tapered
portion of FIG. 10.
[0024] FIG. 12 is a partial microstructure photograph showing the
tapered portion of FIG. 11.
[0025] FIG. 13 is a schematic diagram showing a process of
manufacturing FBAR of the first embodiment.
[0026] FIG. 14 is a schematic diagram showing a process of
manufacturing FBAR of the first embodiment.
[0027] FIG. 15 is a partial microstructure photograph showing a
part of FBAR of the first embodiment.
[0028] FIG. 16 is a schematic diagram showing a process of
manufacturing FBAR of the first embodiment.
[0029] FIG. 17 is a schematic diagram showing a process of
manufacturing FBAR of the first embodiment.
[0030] FIG. 18 is a schematic cross section showing a third
comparative example of the tapered plane of the first
embodiment.
[0031] FIG. 19 is a schematic cross section showing an electronic
device according to a second embodiment of the invention.
[0032] FIG. 20 is a process cross section showing a process of
manufacturing FBAR of the second embodiment.
[0033] FIG. 21 is a process cross section showing a process of
manufacturing FBAR of the second embodiment.
[0034] FIG. 22 is a process cross section showing a process of
manufacturing FBAR of the second embodiment.
[0035] FIG. 23 is a circuit diagram of a voltage control oscillator
mounting the electronic device according to the embodiment.
[0036] FIG. 24 is a schematic diagram showing a mobile phone
mounting the electronic device according to the embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0037] Embodiments of the invention will now be described with
reference to the drawings.
A First Embodiment
[0038] FIG. 1 is a schematic cross section showing an electronic
device according to a first embodiment of the invention.
[0039] FIG. 2A is a top view of the electronic device of the
embodiment, and FIG. 2B is a bottom view thereof.
[0040] In addition, in figures from FIG. 2, elements similar to
those in figures as described before are marked with the same
reference numerals and not described in detail.
[0041] An electronic device in the present embodiment is a thin
film bulk acoustic resonator (FBAR) 5. The FBAR 5 is formed on a
supporting substrate 10 comprising Si (silicon). The supporting
substrate has a cavity portion 60. Furthermore, a thermal oxidation
(SiO.sub.2) film 15 and a lower passivation layer 20 comprising,
for example, silicon nitride (SiN) film are provided in this order
all over the supporting substrate 10. And a first film 30 having a
stacked structure is formed on a main surface of the lower
passivation layer 20. The stacked structure can be formed by
providing an non-crystalline primary layer 27 comprising, for
example, AI.sub.0. 5Ta.sub.0..sub.5, a lower electrode 32
comprising Al and an AIN film 37 comprising aluminum nitride (AIN)
in this order. The crystal of the lower electrode 32 is oriented
along the axis of (111), and that of AIN film 37 is oriented along
the axis of (0001). A first tapered plane 35 and a second tapered
plane 36 are provided respectively at the both end of the first
film 30.
[0042] In the embodiment, lower portions of the first and the
second tapered planes 35, 36 have a curved configuration so that a
slope of the tapered plane becomes gentle with approaching the
supporting substrate 10.
[0043] A second film 40 is provided on the first film 30 except the
second tapered plane 36 and the lower passivation layer 20 on a
side of the first tapered plane 35. The second film 40 is, for
example, a piezoelectric film of AlN. Furthermore, the
piezoelectric film is not limited to being made of AlN, but can be
made of zinc oxide (ZnO) and lead zirconate titanate (PZT). An
upper electrode 50 is provided on the piezoelectric film 40. The
upper electrode 50 can illustratively be made of molybdenum (Mo).
An upper passivation layer 25 is provided on the lower passivation
layer 20, the piezoelectric film 40, the upper electrode 50 and the
second tapered plane 36. An extracting electrode 55 comprising Al
is provided on the upper passivation layer 25.
[0044] The upper electrode 50 and the lower electrode 32 are
connected to the extracting electrode 55 and 55, respectively via a
contact hole.
[0045] Furthermore, the cavity 60 is provided so that the FBAR 5
oscillating in a thickness direction does not touch the supporting
substrate 10. The non-crystalline primary layer 27 and the AlN film
37 have a role to increase the degree of polycrystalline
orientation of the piezoelectric film. The upper and lower
passivation layers 20, 25 have a role to prevent the piezoelectric
film 40 and the non-crystalline primary layer 27 from being
oxidized by atmospheric gases and humidity.
[0046] The piezoelectric film of FBAR 5 expands and contracts in a
direction of thickness on applying a voltage between the upper
electrode 32 and the lower electrode 50. For the application of an
alternative voltage, a vertical resonant oscillation in thickness
is observed at a specified frequency. Moreover a resonant
characteristic is obtained at a desired frequency by adjusting the
film thickness of FBAR 5. For example, where the frequency of 2 GHz
is a pass band, the film thickness of the piezoelectric film 40 is
about 1.5.about.2.0 micrometers, depending on quality of material
and film thicknesses of the upper electrode 50 and the lower
electrode 30. Film thicknesses of the upper electrode 50 and the
lower electrode 30 are 0.2.about.0.3 micrometers. Furthermore, film
thicknesses of the upper and the lower passivation layer 20, 25 are
about 0.1.about.0.05 micrometers. Moreover, assuming that input and
output impedances are, for example, 50 ohm, the shape of the cavity
60 can be a square or a rectangle with a length or width of about
100.about.200 micrometers, respectively. In addition, in the
embodiment in regards to the tapered plane, the passivation layer
and the electrode and the like, those near to the supporting
substrate 10 are "lower" and those far from it are "upper".
[0047] Next, specific examples of the tapered portion will be
described in detail.
[0048] FIG. 3 is a schematic cross section showing a first specific
example of the tapered portion in FIG. 1.
[0049] Moreover, FIG. 4 is a schematic cross section showing a
first comparative example of the tapered portion.
[0050] In addition, the first film 30 will be described here as a
single layer for simplicity.
[0051] First, the first comparative example will be described.
[0052] As shown in FIG. 4, in the comparative example, a first
tapered plane 38 of the first film 30 has a planar shape. Moreover,
an upper end 82 and a lower end 72 of the first tapered plane are
not curved. If the piezoelectric film 40 is formed on the end
portion of the first film 30 like this, "crack" and "fracture" are
more likely to occur on the lower end 72 of the tapered plane 38.
Here, a growth direction of the piezoelectric film 40 is
substantially perpendicular to the main surface of the lower layer.
That is to say, the growth direction (j) on the lower passivation
layer 20 and the growth direction (k) on the first tapered plane 38
run into each other. Where growth directions of the first films 30
run into each other like this, cracks and fractures are likely to
occur.
[0053] On the contrary, according to the first specific example, as
shown in FIG. 3, the lower end 70 of the first tapered plane 35 is
formed in the curved configuration so that the slope of the first
tapered plane 35 becomes gentle with getting close to the
supporting substrate 10. In other words, the first tapered plane 35
has a continuously curved smooth surface facing to the lower end.
Therefore, the growth direction of the piezoelectric film 40 can be
gradually changed near the lower end 70 as shown by an arrow in
FIG. 3. In short, occurrence of cracks and fractures due to running
into each other of two different growth directions each other (for
example, j and k in FIG. 4) can be reduced. As a result, the dense
and continuous piezoelectric film 40 can also be formed on the
lower end 70. As a result of studies by the inventors, it is
revealed that if a curvature radius on the first tapered plane 35
is larger than the thickness of the piezoelectric film formed on
it, cracks and fractures of films due to running into each other of
growth directions of the piezoelectric film 40 formed on it can be
effectively suppressed, and the dense and continuous piezoelectric
film 40 can be formed. That is to say, for the example shown in
FIG. 3, it is advisable that the curvature radius of the first
tapered plane 35 is larger than the thickness of the piezoelectric
film 40, on any place of the first tapered plane 35, although it
changes on every place.
[0054] On the other hand, the growth direction (g) on the first
film 30 and the growth direction (k) on the first tapered plane 35
do not run into each other near the upper end 80 of the first
tapered plane 35 and film growth is made while expanding. That is
to say, the upper portion (k) of the first tapered plane 35 and the
upper portion (g) of the first film 30 grow while filling spacing
of the growth directions, the continuous and dense films are easy
to be formed between those. Studies by inventors indicate that
where the angle .theta. of the end portion 80 is approximately
larger than 135.degree., it is easy to form the continuous and
dense piezoelectric film 40 on it.
[0055] Next, FIG. 5 is a schematic cross section showing a second
specific example of the tapered portion.
[0056] In the specific example, the curved configuration is also
provided on the upper end 80 of the first tapered plane 35, which
the slope of the first tapered plane 35 becomes gentle with getting
close to the upper electrode 50. In this manner, it is possible to
suppress a sharp change of the growth direction of the
piezoelectric film 40 on the upper end 80 and form a more dense and
continuous piezoelectric film 40.
[0057] FIG. 6 is a schematic cross section showing a third specific
example of the tapered portion.
[0058] In the specific example, a convex portion toward to the
piezoelectric film 40 is provided in an intermediate region of the
first tapered plane 35.
[0059] In other word, the first tapered plane 35 is partitioned off
parallel to the lower passivation layer 20. Angles (.theta..sub.1,
.theta..sub.2, . . . .theta..sub.k) between each parallel line and
the first tapered plane 35 are measured. These angles
.theta..sub.1, .theta..sub.2, . . . .theta..sub.k (k is positive
integer) are taken as a function of k. Here, .theta.k has at least
one relative maximum, and the angle increases from .theta..sub.1 to
.theta..sub.n (.theta..sub.1<.theta..sub.2< . . .
<.theta..sub.n) in this order in the downside lower than the
portion giving the relative maximum value .theta..sup.n.
[0060] Providing the relative maximum value on like this makes the
slope gentle with getting close to the lower end 70 of the first
tapered plane 35. Therefore, growth directions of the piezoelectric
film 40 come to experience no running into each other on the lower
end 70, and cracks and fractures of the piezoelectric film 40 can
be suppressed. On the other hand, as growth directions of the
piezoelectric film 40 distribute in diverging directions but not in
directions running into each other near the relative maximum value
.theta..sub.n, cracks and fractures and the like do not occur.
Moreover, .theta..sub.k changes so as to increase gradually with
getting close to the upper end 80 and thereafter to decrease again
in the upper side higher than the portion giving the relative
maximum value .theta..sub.n, therefore the occurrence of cracks and
fractures due to running into each other of growth directions of
the piezoelectric film 40 can be suppressed.
[0061] The structure providing the relative maximum value of
.theta..sub.k in the intermediate region of the tapered plane like
this is effective for the case and so on with the thick first film
30.
[0062] Furthermore, in the specific example, providing the convex
portion in the intermediate region of the first tapered plane 35
makes it easy to increase the angle at the upper end 80 of the
first tapered plane 35. That is to say, as described previously in
FIG. 3, it becomes easy to increase the angle .theta. at the upper
end 80 larger than 135.degree., and then the piezoelectric film 40
without cracks and fractures can be formed on the upper end portion
80, too.
[0063] FIG. 7 is a cross section of the third specific example to
which a film configuration of real FBAR is applied.
[0064] Moreover, FIG. 8 is a schematic cross section showing a
second comparative example of the tapered portion.
[0065] The film configurations of the specific example and the
comparative example are the same as those described previously in
FIG. 1, and have the structure which the lower passivation layer 20
is stacked on a thermal oxidation film 15 and over it the
non-crystalline primary layer 27, the lower electrode 32 and the
AlN film 37 are stacked in this order. In any of these instances,
the first tapered plane 35 is formed from the intermediate to the
upper region of the lower passivation layer 20.
[0066] First, the comparative example will be described.
[0067] Here, an angle of the portion of the lower passivation layer
20 in the first tapered plane 35 is taken as .theta..sub.10.
Moreover, an angle of the portion of the non-crystalline primary
layer 27 provided on the lower passivation layer 20 is taken as
.theta..sub.20.
[0068] As shown in FIG. 8, the comparative example has a structure
with .theta..sub.10, larger than .theta..sub.20
(.theta..sub.10>.theta..sub.20). Where .theta..sub.10 and
.theta..sub.20 are in the relation like this, .theta..sub.10
becomes inevitably large. Therefore, as shown by arrow marks a and
b in FIG. 8, the piezoelectric film 40 grows while the portion
(arrow a) on the major surface of the lower passivation layer 20
and the nearby portion (arrow b) of the first tapered plane 35 are
running into each other. As a consequence, cracks and fractures
become to be likely to occur at the lower end of the first tapered
plane 35.
[0069] On the contrary, in the specific example, as shown in FIG.
7, the angle .theta..sub.1 of the lower passivation layer 20 of the
first tapered plane 35 is smaller than the angle .theta..sub.2 of
the non-crystalline primary layer 27
(.theta..sub.1<.theta..sub.2). In other word, .theta..sub.1
becomes inevitably small. In this manner, it is possible to change
gradually growth directions of the piezoelectric film 40 between
the portion (arrow a) on the major surface of the lower passivation
layer 20 and the nearby portion (arrow b) of the first tapered
plane 35. As a consequence, the growth directions become hard to
run into each other and the piezoelectric film 40 without cracks
and fractures is obtained.
[0070] Next, a relevant part of a method of manufacturing an
electronic device according to the embodiment will be
described.
[0071] FIG. 9, FIG. 10, FIG. 13, FIG. 14, FIG. 16 and FIG. 17 are
process cross sections showing a process of manufacturing the
electronic device according to the embodiment. In addition, the
electronic device is FBAR 5.
[0072] First, as shown in FIG. 9, a thermal oxidation film 15 of
SiO.sub.2 having a film thickness of about 500 nanometers on a
supporting substrate 10 of Si having a substrate thickness of about
600 micrometers. A lower passivation layer 20 of SiN having a film
thickness of about 50 nanometers is formed on the thermal oxidation
film 15 using a plasma CVD (Chemical Vapor Deposition) method. A
non-crystalline primary layer 27 of Al.sub.0.5Ta.sub.0.5 having a
film thickness of, for example, 10 nanometers is deposited on the
lower passivation layer 20 using a sputtering method. A lower
electrode 32 of Al having a film thickness of, for example, about
200 nanometers is deposited on the non-crystalline primary layer
27. Furthermore, an AlN film 37 having a film thickness of 30
nanometers is deposited on the lower electrode 32. Then, after
patterning of a resist mask using photolithography, etching is
performed so as to be a trapezoid narrowing in a direction facing
the supporting substrate 10 using an RIE (Reactive Ion Etching)
method. As a result, a first and a second tapered plane 35, 36 are
obtained at both end portions of a first film 30.
[0073] Here, a method of manufacturing the first and the second
tapered planes 35, 36 will be described below.
[0074] First, the trapezoidal resist mask being in a tapered
configuration at both ends is provided on the first film. For
example, a desired configuration is obtained by heating the resist
to 150.about.200.degree. C. in an oven or on a hot plate after
development. After that, etching is performed by the RIE method.
Then, the first and the second tapered planes 35, 36 of the resist
mask are transferred to the first film 30. In this manner, the
first and the second tapered planes 35, 36 can be formed at both
end portions of the first film 30. Moreover, in the embodiment, the
first and the second tapered planes 35, 36 are formed in the
configuration described previously in FIG. 3, FIG. 5 and FIG.
6.
[0075] The tapered angle of the first film 30 depends on a ratio of
etching rates for the first film 30 and the resist mask. The resist
mask having an etching rate, for example twice higher than that for
the first film 30 is used. As a result, the tapered angle of the
first film 30 can be reduced to about one-half of that of the
resist mask. In the RIE method, for example a mixed gas with
further addition of oxygen gas (.theta..sub.2) after diluting
chlorine gas (Cl.sub.2) and boron trichloride (BCl.sub.3) gas with
argon gas (Ar) can be used.
[0076] Next, as shown in FIG. 10, a piezoelectric film 40 having a
film thickness of 1.7 micrometers is deposited all over the device
comprising the lower passivation layer 20 and the first film 30
using the sputtering method.
[0077] FIG. 11 is a schematic cross section showing the tapered
portion of FIG. 10.
[0078] Directions of slanting lines described in the piezoelectric
film 40 indicate growth directions of a polycrystalline AlN film.
The lower portion 70 of the first tapered plane 35 has a curved
configuration which the slope becomes small with getting close to
the lower passivation layer 20.
[0079] Angles between each layer and the tapered plane 35 are, for
example, 11.degree. for the lower passivation layer 20, 14.degree.
for the non-crystalline primary layer 27, 18.degree. for the lower
electrode 32 and 6.degree. for the AlN film 37. Moreover, it is
known that the angle for the lower electrode 32 is the maximum
value. These angles increase from the lower passivation layer 20
toward the lower electrode 32. This can suppress cracks and
fractures of the piezoelectric film 40 on the lower end 70.
[0080] Moreover, the upper end 80 of the first tapered plane 35
also has a curved configuration which the slope is decreasing
toward the upper electrode direction. The angle for the upper end
is 174.degree.. As the angle for the lower electrode 32 is larger
than those for the AlN film 37 and the non-crystalline primary
layer 27, the convex configuration is formed toward the
piezoelectric film 40 between the upper end 80 and the lower end
70. The configuration can increase the angle for the upper end 80
of the first tapered plane 35. Therefore, cracks and fractures in
the piezoelectric film 40 formed on the upper end 80 become to be
hard to occur.
[0081] FIG. 12 is a TEM (Transmission Electron Microscopy)
observation image showing the tapered portion of FIG. 10.
[0082] According to the embodiment, no cracks and fractures in the
piezoelectric film 40 on the lower end 70 and the upper end 80 are
also confirmed. As for crystalline orientation of the piezoelectric
film 40 located over the lower electrode 30, characterization was
performed via calculation of a half width of a rocking curve
obtained for an AlN (0001) axis using an X-ray diffraction method.
As a result, it was confirmed that the half width for the
piezoelectric film 40 is 1.14.degree. and the film has high
crystalline orientation. The reason that such a highly oriented
piezoelectric film 40 is obtained is that the first film 30 in the
first embodiment comprises the three layers structure made of the
non-crystalline primary layer 27, the lower electrode 32 and the
AlN film 37. The lower electrode 32 on the non-crystalline primary
layer 27 is highly oriented along the (111) axis and the AlN film
37 on it is also highly oriented along the (0001) axis. The
orientation half width for the AlN film (0001) strongly affects
resonant characteristics in a vertical thickness, and for AlN with
a small half width, FBAR 5 having an excellent resonant
characteristics (electric mechanical coupling coefficient kt.sup.2
and Q value) can be obtained. Moreover, a small electric resistance
of the lower electrode 32 allows the electrode to be thin.
Consequently, a ratio of the piezoelectric film 40 in FBAR 5 can be
increased. This results in sufficient use of the excellent AlN
piezoelectric characteristic. However, in order to fabricate a
multilayered film in which each film has a different etching rate
for chlorine gas into a configuration with a smooth and gentle
slope, addition of O.sub.2 gas to the etching gas or excessive
dilution of Cl.sub.2 gas (about 1/100 for Ar gas) is required.
Then, it becomes difficult to optimize the etching condition in
comparison with etching of a normal single film.
[0083] Furthermore, in the embodiment, the relative minimum value
of Rmin of the curvature of the tapered plane 35 is 2.1
micrometers. This is larger than 1.71 micrometers in the film
thickness of the piezoelectric film 40. Therefore, as described
previously in FIG. 3, no cracks and fractures occur in the
piezoelectric film 40.
[0084] According to the embodiment like this, the first film 30 has
a structure which suppresses cracks and fractures in the
piezoelectric film 40 stacked on the first tapered plane 35.
[0085] Subsequently, as shown in FIG. 13, patterning of a resist
mask is performed by photolithography. Then, the piezoelectric film
40 which is deposited on the second tapered plane 36, the lower
passivation layer 20 of the second tapered plane 36 and the lower
passivation layer 20 of the piezoelectric film 40 on the first
tapered plane 35 is removed by the RIE method.
[0086] Next, as shown in FIG. 14, the upper electrode 50 is formed
so as to sandwich the piezoelectric film 40 between the first film
30 and the upper electrode 50. The Mo film 50 with a film thickness
of 300 nanometers is deposited using the sputtering method. Then,
patterning of the resist mask is performed by photolithography.
After that, the second electrode 50 is formed using a method of CDE
(Chemical Dry Etching). At this time, a mixed gas of carbon
fluoride (for example, CF.sub.4) and O.sub.2 may be used.
[0087] FIG. 15 is a TEM image showing a part of FBAR of the first
embodiment.
[0088] It can be confirmed that the upper electrode 50 is provided
all over the piezoelectric film 40. Moreover, it is seen that the
piezoelectric film 40 sandwiched between the first film 30 and the
upper electrode 50 has no cracks and fractures, according to the
structure of the embodiment.
[0089] Subsequently, as shown in FIG. 16, the upper passivation
layer 25 comprising SiN having a film thickness of 50 nanometers is
deposited all over the device by the method of CVD (Chemical Vapor
Deposition).
[0090] Then, contact holes are formed on the lower electrode 32 of
the second tapered plane 36 and on the upper electrode 50 using dry
etching such as RIE or the like, respectively.
[0091] As shown in FIG. 17, an Al film having a film thickness of
1000 nanometers is formed on the upper passivation layer 25 by the
sputtering method. At this time, each electrode and the Al film are
connected through contact holes. Then, patterning of the resist
mask is performed. After that, wet etching is performed using, for
example a mixed solution comprising phosphoric acid, acetic acid
and nitric acid. This results in forming the extracting electrode
55 after selective removal of the Al film of the second tapered
plane and the upper electrode.
[0092] After that, the back side of the Si substrate 10 is dry
etched by a method of Deep-RIE (Deep-Reactive Ion Etching). Then, a
Bosch mode of an ICP-RIE (Inductively Coupling Plasma-RIE) method
may be used as the RIE method, which uses, for example sulfur
hexafluoride (SF.sub.6) and carbon fluoride (for example,
C.sub.4F.sub.8) gases. In the Bosch mode, SF.sub.6 gas plays a role
to etch Si. C.sub.4F.sub.8 gas plays a role to form a polymer
protective film on a Si side wall formed during etching. Therefore,
alternative supply of these gases allows the Si substrate 10 to be
etched substantially vertically, and to give the cavity 60 with a
desired size.
[0093] This results in the removal of the Si substrate 10 under the
first film 30. Moreover, the thermal oxidation film 15 is removed
using, for example, solution of antimony fluoride. Then, the cavity
60 is formed under the first film 30. In this manner, the relevant
part of FBAR 5 shown in FIG. 1 is completed.
[0094] On the contrary, a comparative example will be described
below.
[0095] FIG. 18 is a schematic cross section showing the comparative
example.
[0096] In the comparative example, for taper fabrication of the
first film 30, the resist patterned by photolithography as well as
in the first embodiment is baked at a high temperature. Then, the
fabrication is performed by the RIE method, using the resist mask
of which the end portion is fabricated into a tapered
configuration. In the first embodiment, as the etching gas used in
the RIE method, Cl.sub.2 gas and BCl.sub.3 gas are diluted with Ar
and O.sub.2 gas is added to them. However, in the comparative
example, BCl.sub.3 gas was increased, for example, about twice as
in the first embodiment, furthermore etching was performed without
addition of O.sub.2 gas.
[0097] Consequently, angles between each film and the first tapered
layer 35 are, for example, 80.degree. for the lower passivation
layer 20, 36.degree. for the non-crystalline primary layer 27,
30.degree. for the lower electrode 30 and 40.degree. for the AlN
film 37. That is to say, as the taper angle of the lower
passivation layer 20 is substantially perpendicular to the major
surface of the supporting substrate 10, growth directions of the
piezoelectric film 40 run into each other. Therefore, it is seen
that cracks and fractures are formed in the piezoelectric film 40
on the lower end 70.
[0098] Furthermore, on the first tapered plane 35, occurrence of
cracks and fractures was also confirmed at the interface between
the lower electrode 32 and the AlN film 37. This is because growth
directions of the piezoelectric film 40 run into each other due to
the taper angle of the lower electrode 32 being smaller than that
of the AlN film 37.
[0099] The extracting electrode 55 was formed using wet etching
fabrication on this sample as well as the first embodiment.
However, the etchant infiltrates through from cracks and fractures
of the lower end 70, and the lower electrode 32 was etched.
Therefore, desirable characteristics were not obtained due to
decrease of the resonant area. Moreover, occurrence of cracks and
fractures near the back side of the first film 30 was observed.
A Second Embodiment
[0100] FIG. 19 is a schematic cross section showing an electronic
device according to a second embodiment of the invention.
[0101] The electronic device of the embodiment is also FBAR (Thin
Film Bulk Acoustic Resonator) 5. The FBAR 5 is formed on a
supporting substrate 110 of Si. The supporting substrate 110 has a
cavity 160. Then, all over the supporting substrate 110, a thermal
oxidation (SiO.sub.2) film 115 and a lower layer 120 of AlN are
provided in this order. The lower layer 120 is crystalline oriented
along a (0001) axis. A half width of a rocking curve of the (0001)
axis by an X-ray diffraction method is about 10.degree.. The first
film is provided on the lower layer 120. In the embodiment, the
first film is the lower electrode 132 of Mo.
[0102] The lower electrode 132 is in a trapezoid configuration
narrowing toward an upper electrode 150. Moreover, both ends of the
lower electrode 132 are provided with a first tapered plane 135 and
a second tapered plane 136, respectively.
[0103] A piezoelectric film 140 made of, for example, AlN is
provided over the lower electrode 132 selectively including the
second tapered plane 136 toward the first tapered plane 135 and the
lower layer 120 on the side of the first tapered plane 135.
Furthermore, the piezoelectric film is not limited to being made of
AlN, but can be made of ZnO and PZT. Over the piezoelectric film
140, the upper electrode 150 is provided. An upper passivation
layer 125 and an extracting electrode 155 of Al are provided over
the lower layer 120, the piezoelectric film 140, the upper
electrode 150 and the second tapered plane 136. The upper electrode
150 and the lower electrode 132 have a selective contact hole,
respectively. The upper electrode 150 and the lower electrode 132
are connected to the extracting electrode 155 through contact
holes, respectively.
[0104] In the embodiment, lower ends 170 of the first and the
second tapered planes 135, 136 are curved so that the slope of the
tapered plane becomes gentle with getting close to the lower layer
120. This causes cracks and fractures hard to occur in the
piezoelectric film 140 stacked on the end portion of the first
tapered plane 135 of the lower electrode 132, and the excellent
piezoelectric film 130 is obtained.
[0105] Hereinafter, a method of manufacturing FBAR 5 of the second
embodiment shown in FIG. 19 will be described.
[0106] Here, FIG. 20.about.FIG. 22 are process cross sections of a
process of manufacturing FBAR of the second embodiment.
[0107] First, as shown in FIG. 20, a thermal oxidation film 115
comprising SiO.sub.2 with a film thickness of about 300 nanometers
is formed on a supporting substrate 110 of Si with a substrate
thickness of about 600 microns. The lower layer 120 comprising AlN
with a film thickness of about 30 nanometers is formed on the
thermal oxidation film 115 using the sputtering method. The lower
layer 120 is crystalline oriented to the (0001) axis. A Mo film
with a film thickness, for example, of 300 nanometers is
continuously deposited on the lower layer 120 using the sputtering
method.
[0108] Moreover, after a patterning of a resist mask by
photolithography, an etching is performed so that the lower
electrode 132 is in a trapezoid configuration narrowing toward the
direction facing the supporting substrate 110. This provides both
ends of the lower electrode 132 with the first and the second
tapered planes 135, 136.
[0109] At this time, mixed gases, for example, of carbon fluoride
(for example CH.sub.4) and O.sub.2 gas can be used. Furthermore,
the lower electrode 132 is etched while changing gradually a ratio
CF.sub.4/)O.sub.2 in the mixed gas. Then a configuration with a
tapered plane slope gradually curved with getting close to the
lower layer 120 is formed. Moreover, AlN used for the lower layer
120 is resistant to the mixed gas, thereby plays a role as a
stopper layer.
[0110] Subsequently, as shown in FIG. 21, the AlN film with a film
thickness of 1.16 micrometers is deposited over the lower layer 120
and the lower electrode 132 using the sputtering method. Then,
patterning of a resist mask is performed by photolithography. The
AlN film on the lower layer is removed so as to enclose the lower
electrode 132 by the RIE method using a mixed gas of Cl.sub.2 and
BCl.sub.3. However, the AlN film on the second tapered plane 136 is
removed for connection to the extracting electrode 155. Then, the
piezoelectric film 140 is formed. In this manner, providing the AlN
film of the lower layer 120 under the lower electrode 132 can
improves the orientation of the lower electrode 132. Therefore, the
orientation of the (0001) axis in the piezoelectric film 140 can be
improved by setting the lower electrode 132 to be the
substrate.
[0111] For example, it is difficult to form the highly oriented Mo
film on the supporting substrate 110 of Si or SiO.sub.2. Like the
embodiment, the orientation of the Mo film is about 2.0.degree.,
even if the lower layer 120 of AlN with the thickness of about 30
nanometers is provided. Therefore, the orientation half width of
the (0001) axis is about 2.0.degree., although the AlN film is
formed on the Mo film. Moreover, Mo has a higher electrical
resistance compared with Al of the first embodiment, then, causing
Mo to be a thin film results in a higher serial resistance and a
lower Q value.
[0112] However, the lower electrode 132 is made of a single layer
film of Mo, and it needs to change a mixed ratio of etching gases
during etching for processing it into a gentle and gradual slope
configuration. But, it can be achieved by a relatively simple
etching apparatus such as CDE (Chemical Dry Etching) or the like.
Therefore, the FBAR characteristic of the Al lower electrode of the
first embodiment is superior, but the process is simple and the
same electrode is used for the upper and the lower ones, and from
viewpoints of savings of a process chamber for sputtering film
formation and sputtering targets, the second embodiment gives a
more effective device structure.
[0113] The observation of the first tapered portion 135 of the
lower electrode 132 revealed that the angle of the upper end 180 of
the first tapered portion 135 is 145.degree.. Moreover, it was
confirmed that cracks and fractures do not occur in the
piezoelectric film 140 on the lower end 170 and the upper end
180.
[0114] Furthermore, a curvature radius of each tapered plane 135
partitioned off parallel to the supporting substrate 110, for
example, every 10 nanometers was measured. As a result, the minimum
value Rmin of the curvature was 1.8 micrometers. This value is
larger than 1.16 micrometers of the film thickness of the
piezoelectric film 140. Therefore, it is revealed that no cracks
and fractures occur in the piezoelectric film 140 on the first
tapered plane 135, as described previously in FIG. 3.
[0115] Furthermore, partitioning off into 5 layers is performed
every 65 nanometers of particle size of the piezoelectric film 140
parallel to the supporting substrate 110. For example, they are
four layers from the lower layer 120 toward to the upper electrode
150 and the other residual one layer. Then, angles between each
layer and the first tapered plane 135 (.theta..sub.1,
.theta..sub.2, .theta..sub.3, .theta..sub.4, .theta..sub.5) were
measured. Angles were 12.degree. for .theta..sub.1, 18.degree. for
.theta..sub.2, 23.degree. for .theta..sub.3, 28.degree. for
.theta..sub.4 and 35.degree. for .theta..sub.5, respectively toward
to the upper electrode 150. It is revealed that the maximum angle
is .theta..sub.5 and the angle increases from .theta.1 to .theta.5
in this order
(.theta..sub.1<.theta..sub.2<.theta..sub.3<.theta..sub.4&l-
t;.theta..sub.5). The relation like this causes the lower end 170
of the first tapered plane 135 to be in a configuration that the
slope becomes small with getting close to the lower layer 120.
Then, cracks and fractures can be suppressed in the piezoelectric
film 140.
[0116] Thereafter, the Mo film with a film thickness of 300
nanometers is deposited on the piezoelectric film 140 using the
sputtering method. Then, the selective resist patterning is
performed by photolithography. Moreover, the upper electrode 150 is
formed by etching using the CDE (Chemical Dry Etching) method.
[0117] Subsequently, as shown in FIG. 22, the upper passivation
layer 121 is formed by depositing the SiN film with a film
thickness of 50 nanometers all over the device using the sputtering
method. Thereafter, contact holes are formed in the upper electrode
150 on the side of the second tapered plane 136 and on the side of
the first tapered plane 135.
[0118] Furthermore, the Al film with a film thickness of 1000
nanometers is deposited on the upper passivation layer 125 using
the sputtering method. Patterning of the resist mask is performed
by photolithography. Thereafter, wet etching is performed using a
mixed solution including, for example phosphoric acid, acetic acid
and nitric acid. This results in formation of the extracting
electrode 155 by selective removal of the Al film on the lower
electrode 132 and the upper electrode 150. The extracting electrode
155 is connected to the lower electrode 132 and the upper electrode
150 through contact holes, respectively.
[0119] Furthermore, as shown in FIG. 19, the back side of the
supporting substrate 110 is etched by a dry process using the
method of Deep-RIE (Deep Reactive Ion Etching). Thus, the
supporting substrate 110 under the lower electrode 132 is removed.
Moreover, the thermal oxidation film 115 is removed using, for
example ammonium solution. Then, a cavity 160 is formed on the back
side of the lower electrode 132. In this way, FBAR 5 of FIG. 19 is
completed.
[0120] On the contrary, a comparative example of the second
embodiment will be described below.
[0121] First, a first comparative example is described.
[0122] That is to say, the basic structure of the comparative
example is substantially the same as the second embodiment.
However, the first and the second tapered planes 135, 136 were
formed by a process of both ends of the lower electrode 132 using a
mixed gas including CF4 gas with a high concentration. In the
process, the composition of the mixed gas was kept constant.
[0123] As a result, the first tapered plane 135 came into a flat
configuration as described previously in FIG. 4. Angles at the
lower end 170 and the upper end 180 of the tapered portion (see
FIG. 19) were 25.degree. and 155.degree., respectively. It was
confirmed that cracks and fractures originating from the lower end
170 of the first tapered plane 135 occur in the piezoelectric film
140 on the tapered portion.
[0124] Next, a second comparative example is described.
[0125] In the comparative example, the formation of the first and
the second tapered planes 135, 136 of the lower electrode 132 was
performed by wet etching using mixed solution comprising acetic
acid, phosphoric acid and nitric acid. The use of this mixed
solution allows isotropic etching to be achieved.
[0126] At the lower end 170 of the first and the second tapered
planes 135, 136, the slope of the tapered plane becomes gentle with
getting close to the lower layer 120.
[0127] However, an angle at the upper end 180 of the first tapered
plane 135 was 130.degree.. Furthermore, a curvature radius of each
tapered plane 135 partitioned off parallel to the lower layer 120,
for example every 10 nanometers was measured. As a result, the
minimum value Rmin of the curvature was 1.75 micrometers and larger
than 1.16 micrometers of the film thickness of the piezoelectric
film 140. Therefore, in the comparative example, the substantially
continuous piezoelectric film 140 was obtained at the lower end 170
of the first tapered plane 135 or on the tapered plane 135, but it
was revealed that cracks and fractures occur in the piezoelectric
film 140, originating from the upper end 180 of the first tapered
plane 135.
[0128] Next, a third comparative example is described.
[0129] In the comparative example, for the formation of the first
and the second tapered planes 135, 136 of the lower electrode 132,
mixed gas comprising CF.sub.4 gas and O.sub.2 gas was used as
etching gas. In the process, etching was performed, while the
CF.sub.4/O.sub.2 ratio in the mixed gas was varied in three steps
different from a consecutive change like the second embodiment. For
example, they are the first gas mixed ratio and the last mixed
ratio in the second embodiment, and the intermediate ratio between
them.
[0130] This result allowed the curved configuration to be obtained
at the lower end 170 of the first and the second tapered planes
135, 136, which the slope of the tapered plane becomes gentle with
getting close to the lower layer 120. Moreover, the angle at the
upper end 180 of the first tapered plane 135 was 140.degree..
However, a curvature radius of each tapered plane 35 partitioned
off parallel to the lower layer 120, for example every 10
nanometers was measured. As a result, it was revealed that the
minimum value Rmin of the curvature was 1.0 micrometers and smaller
than 1.16 micrometers of the film thickness of the piezoelectric
film 140. Thus, cracks occurred in the piezoelectric film 140. This
is because that the curved configuration of the tapered plane
became sharp and the curvature radius became small due to the three
steps change of gas ratio during etching the lower electrode 132,
in comparison with the consecutive change.
[0131] Embodiments of the invention have been described with
reference to embodiments and comparative examples.
[0132] A frequency filter can be manufactured by combining plural
FBARs with different resonant frequency in parallel or in series
using FBAR 5 such as FBAR 5 shown in the first and the second
embodiment. For example, a frequency filter of 2 GHz zone is
obtained by lowering the resonant frequency of the parallel FBAR by
about 70 MHz than the resonant frequency of the serial FBAR.
[0133] It can be formed on the supporting substrate 110 of
semiconductor as with the first and the second embodiments.
Therefore, for example, it is also easy to make an RF filter to be
monolithic. Moreover, according to the embodiment, an excellent
characteristic and high-efficiency FBAR filter 100 is supplied.
[0134] FIG. 23 is a circuit diagram of a voltage controlled
oscillator mounting an electronic device according to the
embodiment.
[0135] The Voltage Controlled Oscillator (VCO) 122 has FBAR 5, an
amplifier 126, a buffer amplifier 130 and capacitance variable
capacitors C1, C2. Here, the feedback of frequency components
passing through the FBAR filter 100 is made to the amplifier 126,
and output signal is taken. Thereby, it allows the frequency
adjustment to be achieved.
[0136] The VCO 122 like this contributes to downsizing due to its
simple constitution. For example, it is mounted on a cellular phone
as shown in FIG. 4 and information terminal devices such as PDA and
a notebook PC not shown.
[0137] Embodiments of the invention have been described with
reference to examples. However, the invention is not limited to
these examples. For example, even if any shape except a square,
that is, a quadrangle such as a rectangle, a triangle, a polygon
and an inequilateral polygon or the like are used for the planar
shape of oscillating portion in FBAR of the embodiment, more of the
same effects as the embodiment are obtained.
[0138] Moreover, in the embodiment, silicon was used for the
supporting substrate material, but for example, gallium arsenide
(GaAs), indium phosphide (InP), quartz, glass or other materials
such as plastic having heat resistance of about 200.degree. C. can
also be used.
[0139] Furthermore, FBAR was described as an electronic device of
the invention, but the invention is not limited to this, and more
of the same working effects are obtained from a similar embodiment
about other electronic devices such as MEMS device.
[0140] The material, composition, shape, pattern and manufacturing
process of elements constituting the electronic device of the
invention that are adapted by those skilled in the art are also
encompassed within the scope of the invention as long as they
include the features of the invention.
* * * * *